Fuel cells are a kind of power generator which extracts electric energy through electrochemical oxidation of fuels such as hydrogen and methanol. In recent years, the fuel cells have drawn attention as a clean energy supply source. Among fuel cells, polymer electrolyte fuel cell is operated at a low standard working temperature of approximately 100° C., and provides high energy density, and thus the polymer electrolyte fuel cell is expected to be widely applied as relatively small-scale distributed power facilities and as mobile power generator on automobile, ship, and the like. In addition, the polymer electrolyte fuel cell also draws attention as power source of small-scale mobile apparatus and portable apparatus, and is expected to be mounted on cell phone, personal computer, and the like in place of secondary battery such as nickel-hydrogen battery and lithium-ion battery.
A normal fuel cell is constituted by cell units, the cell unit having a configuration of a membrane electrode assembly (hereinafter referred to also as MEA) being sandwiched between separators, which MEA is constituted by an anode electrode and a cathode electrode in which a reaction of power generation occurs, and by a polymer electrolyte membrane serving as a proton conductor between the anode and the cathode. Although the main component of the polymer electrolyte membrane is an ionic group-containing polymer, (polymer electrolyte material), there can also be used a polymer electrolyte composition containing an additive and the like in order to increase the durability. The polymer electrolyte composition is also suitable for the binder and the like of the electrode catalyst layer being used in a specifically severe oxidizing atmosphere. The characteristics required of the polymer electrolyte membrane and the polymer electrolyte composition include, first, high proton conductivity, specifically high proton conductivity even under high temperature and low-humidification conditions. Since the polymer electrolyte membrane and the polymer electrolyte composition also function as the barrier that prevents direct reaction between fuel and oxygen, low permeability of fuel is required. Other necessary characteristics include chemical stability for withstanding strong oxidizing atmosphere during operation of fuel cell, mechanical strength and physical durability of being capable of withstanding thinning of membrane and repeated swell-drying cycles.
Conventionally, as the polymer electrolyte membranes, there is widely used Nafion (registered trade name, manufactured by DuPont) which is a perfluoro-sulfonate-based polymer. Since Nafion (registered trademark) is manufactured through multistage synthesis, it has a problem of extremely expensive and large fuel-crossover (transmission amount of fuel). In addition, as to Nafion, there were pointed out a problem of losing membrane mechanical strength and physical durability by swelling-drying, a problem in which the use at high temperatures is not possible because of low softening point, a problem of waste disposal after use, and further an issue of difficulty in recycling the material. The development of hydrocarbon-based electrolyte membranes has been actively conducted in recent years as a polymer electrolyte membrane having excellent membrane characteristics at a low price and being capable of substituting Nafion (registered trademark).
However, these polymer electrolyte membranes have a problem of insufficient chemical stability in the case of the use of polymer electrolyte fuel cells. Although the mechanism of chemical deterioration has not fully been clarified, hydrogen peroxide and hydroxy radical, generated during power generation, break the polymer chain and the side chain, resulting in thinning and weakening of the polymer electrolyte membrane. In addition, during repeated swelling and shrinking in association with changes in humidity, there has been a problem in which the weakened polymer electrolyte membrane breaks and thus power generation does not become possible.
In the above situation, there have been conducting studies to improve the chemical stability and improve the durability by using a polymer electrolyte composition applying perfluoro-based electrolyte membrane and hydrocarbon-based electrolyte membrane each containing antioxidant.
For example, Patent Literatures 1 and 2 propose polymer electrolyte compositions adding an antioxidant such as phosphorous acid ester (phosphite), thioether, hindered amine, and hindered phenol to a sulfonic acid group-containing polyethersulfone-based polymer or a sulfonic acid group-containing polyarylene-based polymer. Patent Literature 3 provides a polymer electrolyte composition adding a phosphorous acid ester (phosphite)-based antioxidant to a sulfonic acid group-containing polyethersulfone-based polymer.
Furthermore, Patent Literature 4 proposes a polymer electrolyte composition adding a phosphonic acid group-containing polymer such as polyvinylphosphonic acid to a sulfonic acid group-containing polyethersulfone-based polymer or a sulfonic acid group-containing polyetherketone-based polymer.
In addition, Patent Literature 5 proposes a polymer electrolyte composition adding cerium ion or manganese ion to a perfluoro sulfonic acid-based polymer and a sulfonic acid group-containing polyetherketone-based polymer.